Oxidases showing oxidization actions on various substrates such as phenolic compounds and polyphenolic compounds can be mainly classified into two groups of peroxidases and phenol oxidases.
The above-mentioned peroxidases catalyze oxidation of various substrates and require the presence of hydrogen peroxide as a common substrate in a reaction system. On the other hand, the phenol oxidases catalyze oxidation of various substrates and require the presence of molecular oxygen as a common substrate. Therefore, among known oxidases, since the above-mentioned phenol oxidases can catalyze oxidation of various substrates in the presence of oxygen in the air, the phenol oxidases are suitable for diversified chemical reactions such as coloring, decolorization, polymerization and degradation caused by generation of radical species as intermediates in the presence of oxygen.
The above-mentioned phenol oxidases have various catalytic abilities based on the action of radical species formed as reaction intermediates. For instance, it has been disclosed that when phenothiazine-10-propionic acid is used as a mediator, the degradation reaction of indigo can be efficiently carried out with laccase, which is a phenol oxidase (2001, University of Fukui, Center for Cooperative Research in Science and Technology, “Materials on Workshop for High-Technology,” p. 55).
The above-mentioned oxidases have conventional been found in various plants, bacteria, fungi and the like. For instance, in the plants, the above-mentioned oxidases have been found in secretory pipe of Anacardiaceae, peaches, chestnuts, and those belonging to Podocarpaceae. In the fungi, the above-mentioned oxidases have been found in Aspergillus, Botrytis, Myrothecium, Penicillium, Pestalotia, Rhizoctonia and the like belonging to the genus Deuteromycotina; Pleurotus, Lentinus, Polyporus, Trametes, Coriolus and the like belonging to the genus Basidiomycotina; Podospora, Neurospora, Monocillium and the like belonging to the genus Ascomycotina. In the bacteria, the above-mentioned oxidases have been found in Bacillus, Azospirillum, Streptomyces, Aerobacter and the like. In addition, in the edible mushrooms, which are Basidiomysetes, the above-mentioned oxidases have been found in, for instance, Schizophyllum commune, Coriolus versicolor, Pycnoporus coccineus, Pleurotus octreatus, Fomitella fraxinea and the like.
However, since many phenol oxidases possess the optimum pH near an acidic pH due to their limited uses. In addition, phenol oxidases having the optimum pH in the neutral to alkaline pH may not efficiently act on various substrates near the neutral pH because their optimum pHs are greatly changed depending upon the substrates used.
On the other hand, Flammulina velutipes has been found to exhibit laccase activity by culturing Flammulina velutipes in a medium at a pH of 6.0 (Japanese Patent Laid-Open No. Sho 60-156385).
However, the above-mentioned laccase is an enzyme having the optimum pH on the acidic pH, and has a defect that the activity is lowered in a pH of the neutral to alkaline region.